1. Field
The invention relates to the multiplexed transmission of wideband signals and, more particularly, to the generation of an improved modulation format, and to the transmission of multiple wideband video signals from a video camera via a single optical fiber.
2. Prior Art
Traditionally, wideband signals, such as color video signals generated by a broadcast television camera, are transmitted from the camera to the central processing unit via a multiple conductor (coaxial) cable. Such scheme requires massive amounts of equalization to correct for the losses in the cable. The equalization has to be switched in and out depending upon the length of the cable used. Disadvantageously, even the very best equalizers produce a large amount of unexceptable distortion. Additionally, the cable itself is bulky and cumbersome to handle.
In a more sophisticated approach, the wideband signals are transmitted via a triax system, which involves modulating the multiple wideband signals on various AM or FM carriers and transmitting them via a single, very large diameter coaxial cable. This scheme requires complex and expensive design work, with a corresponding use of expensive hardware to implement.
In a third approach, a fiber optical cable may be used to derive the advantage that much larger distances may be realized with minimum distortion and with extremely wide signal bandwidth capabilities. However, for a straightforward application, multiple signals require multiple fibers within the cable, with corresponding multiple connectors at either end thereof. This is advantageous, since optical fibers are presently very expensive. In addition, two copper wires are required for power transmission. Such a multiple optical fiber cable configuration is therefore also undesirable.
Thus, it is preferable to transmit the multiple signals via a single optical fiber cable to minimize the expense and the number of optical fibers and connectors. In one scheme using a single optical fiber cable, one signal is transmitted as a pure baseband signal, and the remaining signals are transmitted on some given type of FM or AM carriers. However, the light generating laser diode generally used in such a scheme is rather a non-linear device which tends to generate massive intermodulation distortion, i.e., excessive crosstalk between channels, unless the relative levels of the carriers are carefully limited. This in turn reduces the carrier levels, which undesirably degrades the signal-to-noise ratio.